Ice ages of the Pleistocene

1. Ice ages of the Pleistocene • There is field evidence for glaciation • Major northern hemisphere glaciation began about 2.8 Ma • Ice age climates were colder because • Greater albedo (reflectance) • Greater ice albedo • Greater land albedo (less vegetation) • Lower atmospheric greenhouse gas concentrations • Lower CO2 • Lower CH4 and N2O • Pleistocene climate changes were paced by variations in Earth’s orbit

2. Wisconsin drumlin (In what direction would glacier have been flowing?) Field evidence for glaciation Drumlin swarm: note parallel orientation Glacier carved valley in western U. S.

3. Field evidence for glaciation (cont.) Glacial erratic, N. Dakota Glacial striations Glacial erratic, Alberta, Canada

4. Field evidence for glaciation (cont.): Glacial moraines (rockpiles bulldozed by the glaciers) Long Island elevation Harbor Hill moraine Ronkonkama moraine Glacial till (clay, sand, and small rocks) of the Ronkonkama Moraine, Long Island Lateral (side) and end moraines produced by a glacier on Baffin Island, Canada

5. Nature of the ice age Earth • At glacial maximum, large parts of Eurasia and North America were ice-covered • Sea level was ~ 130 m lower • Earth’s temperature was ~ 6˚C colder • Snowlines were ~ 1000 m lower (glaciers came ~ 1000 m lower down midlatitude and tropical mountains) • Biogenic greenhouse gas concentrations were lower (CO2, CH4, and N2O) • Continents were considerably drier • In ice-free areas, there were fewer forests and more grasslands and deserts

6. Why was glacial Earth colder? • More extensive ice sheets, higher ice albedo (reflectance) • More desert and less forested land on continents, higher albedo • Lower concentration of greenhouse gases Estimated contributions to cooler glacial climates: Higher albedo 60% (~ 4˚C cooling) Lower greenhouse gases 40 % (~ 2˚C cooling)

7. Northern hemisphere glaciation begins around 2.8 Ma • Glacial detritus appears in North Pacific sediments • 18O of benthic forams becomes heavier (more positive) • At later times: • Earth cools more • Cyclic climate change • Amplitude gets larger • 1 ‰ = 4˚ C or 100 m sea level Sir Nicholas Shackleton

8. History of glaciation, last 5 Ma: 18O of CaCO3 plotted vs. time, 5 Ma to present 0 - 1.8 Ma 1.8 - 3.6 Ma 18O of CaCO3 (‰) (colder, more ice) 3.6 - 5.4 Ma Lisiecki and Raymo, 2005 Time before present, 103 years

9. History of glaciation, last 5. Ma: 18O of CaCO3 plotted vs. time, 5 Ma to present 0 - 1.8 Ma 1.8 - 3.6 Ma 18O of CaCO3 (‰) (colder, more ice) Cold times are colder after 2.8 Ma Northern hemisphere glaciation begins ~ 2.8 Ma 3.6 - 5.4 Ma Lisiecki and Raymo, 2005 Time before present, 103 years

10. Cyclicity of ice ages, last 3.6 Ma Age, ka (kyr years)

11. How do we know glacial greenhouse gas concentrations? Study bubbles of trapped air in polar ice cores Paul Mayewski, U. S. Summit, Greenland (GISP2) Jean Jouzel, France CO2 during interglacials: 280 ppm CO2 during glacials: 190 ppm CH4, N2O: higher during interglacial periods than glacial Claude Lorius, Jean-Robert Petit, France. Vostok, East Antarctic Plateau

12. CO2 Temperature, CH4, and CO2 records from the Vostok ice core Isotopic temp of ice CH4 Foram 18O 18O of CaCO3 (‰) (colder, more ice) 0 ka 200 ka 400 ka 600 ka 800 ka

13. CO2 Temperature, CH4, and CO2 records from the Vostok ice core Isotopic temp CH4 Foram 18O 18O of CaCO3 (‰) (colder, more ice) 0 ka 200 ka 400 ka 800 ka

14. More fertile oceans mean less CO2 in the atmosphere Atmosphere • Where is CO2 on the surface today? • Atmosphere 600 Gt C (1 Gt = 1015 grams C) • Land biosphere 600Soils 1,500 • Dissolved CO2 in oceans (mostly HCO3-) 38,000 • Therefore: glacial atmospheric CO2 was lower because something changed in the oceans • Two hypotheses for lower glacial CO2 concentrations: • Oceans were more alkaline (higher pH) • CO2 + OH- ---> HCO3- • or • Oceans were more fertile, ocean plants took up more CO2 CO2 Surface ocean CO2+H2O ---> CH2O+O2 Low CO2 Deep ocean CH2O+O2 ---> CO2+H2O High CO2 Wallace Broecker

15. Ocean circulation and CO2 Ocean surface • Organic matter sinks into deep ocean • Respiration releases CO2 to deep waters • Deep waters come to the surface in the Southern Ocean; CO2 escapes to atmosphere • During glacial times, ice or low density fresh water is present at the surface • Deep waters come to the surface more slowly • CO2 gets “stuck” in deep ocean waters • Atmospheric CO2 falls Sea floor 80˚S 60˚S 30˚S

16. What paces cycles of Pleistocene climate change? Milutin Milankovitch • Periodic changes in Earth’s orbit about the sun: • Tilt of Earth’s spin axis. • Period of 41 kyr • Eccentricity of Earth’s orbit. • Period of ~ 100 kyr • Precession (wobble) of Earth’s spin axis • Period of ~ 23 kyr • Changes in Earth’s orbit change heating between summer and winter

17. What paces cycles of Pleistocene climate change? 1. Most ice sheets are in the northern hemisphere. 2. Warm northern hemisphere summers melt ice sheets; cool northern summers grow ice sheets • Periodic changes in Earth’s orbit about the sun: • Tilt of Earth’s spin axis. • Period of 41 kyr • Eccentricity of Earth’s orbit. • Period of ~ 100 kyr • Precession (wobble) of Earth’s spin axis • Period of ~ 23 kyr • Changes in Earth’s orbit change heating between summer and winter http://physics.weber.edu/palen/Clearinghouse/labs/Seasons_pl/seasons.html

18. Effect of tilt on seasonality: Higher tilt means more sunlight at mid-high latitudes during summer, means warmer summers N N S S Low tilt High tilt http://physics.weber.edu/palen/Clearinghouse/labs/Seasons_pl/seasons.html

19. Effect of eccentricity: When Earth’s orbit is circular, the position on the orbit where a season occurs does not affect climate. When the orbit is more elliptical, the planet will be warmer during the season it is close to the sun, and cooler during the season when it is farther from the sun.

20. Effect of precession 1. Northern hemisphere far from sun in summer: cool summers SUN Earth’s spin axis 2. Northern hemisphere close to sun in summer: warm summers SUN

21. What caused the ~ 40 kyr cycles between 1 - 2 Ma (“40 k world”)?Peter Huybers’ theory • One theory – a linear response to obliquity (tilt) changes • At low tilt • Northern hemisphere summers are cool • Ice sheets grow • At high tilt • Northern hemisphere summers are warm • Ice sheets melt • Why in there no precession signal in the “40 k world”? • When Northern hemisphere is close to the sun in summer • Summers are hot but… • Earth moves quickly, summers are short (Kepler’s law) • When Northern Hemisphere is far from sun in summer • Summers are cool but… • Summers are long • Short, warm summers and long, cool summers cancel each other

22. 1 idealized cycle What caused the 100 kyr cycles? A hypothesis Milankovitch fluctuations Long-term cooling, ice growth 18O of CaCO3 (‰) Rapid warming, melting How do you have overshoot? What causes warming? • Ice grows during cold N. hemisphere summers • Ice melts slowly during warm N. summers • Ice sheet frozen to the continent • When ice sheet thickness is very big, geothermal heating melts the bottom • Ice slides along bedrock • Next warming, entire ice sheet melts! Overshoot ! Age, ka (kyr) Time Small ice sheet, poor insulator Cold Geothermal heat Cold Large ice sheet, good insulator Geothermal heat

23. What happens at glacial terminations? • ? Something happens in the Northern Hemisphere that changes ocean circulation around Antarctica • CO2 rises • Earth warms • Northern hemisphere ice sheets melt • Albedo decreases • Westerlies over the Southern Ocean shift south • CO2 rises • And so on…

24. Rapid climate change events; links between high and low latitudes-1 • Rapid climate change events: rapid warming, slow cooling in Greenland • Antarctica warms before Greenland, cools after Greenland warms • CH4 rises when Greenland warms: more swamps and bogs in warmer N hemisphere climate • CO2 rises when Antarctica warms: faster vertical mixing, CO2 released from deep water 10 ka 40 ka 90 ka

25. Rapid climate change events; links between high and low latitudes-2 • When Greenland is warm: • Asian monsoons are stronger • Changes in ocean and atmosphere circulation affect equatorial Atlantic, equatorial Pacific, other areas 10 ka 40 ka 90 ka

26. Low latitude climate change: the sun is everythingIsotopic composition of an eastern Chinese speleothem Insolation 10 ka 40 ka Wang et al., 2008 0 ka 100 ka 200 ka